Microwave holographic measuring method and apparatus

ABSTRACT

A microwave h lographic measuring method is disclosed. A first electrical signal S f microwave frequency is provided. A first part S 1  of the first signal S is directed t a first antenna ( 101 ). Predetermined changes of phase and amplitude are applied to a second part S 2  of the first signal S to produce a second electrical signal S 4  which is coherent with the first part S 1  of the first signal S. Microwave radiati n is detected a t plurality of locations by means of a second antenna ( 107 ) to generate a third electrical signal S 5  at each location. The second S 4  and third S 5  electrical signals are combined t produce a fourth electrical signal.

[0001] The present invention relates to a microwave holographicmeasuring method and apparatus, and relates particularly, but notexclusively, to such method and apparatus for testing a microwaveantenna or forming an image of an object.

[0002] Methods of testing microwave antennas by microwave holography areknown and an arrangement for carrying out such a method is shown inFIG. 1. A signal from a network analyser 1 causes an antenna 2 undertest to emit microwave radiation, which is sampled at suitable intervalsover an appropriate aperture by means of a sampling antenna 3 which canbe moved along the axes X and Y shown in the figure. The output signalfrom the sampling antenna 3 is returned to the network analyser. At eachsampling position, the signal radiated from the antenna 2 under test isfed to the network analyser 1, which determines the relative amplitudeand phase of the return signal by comparison with the output signal ofthe network analyser. These values of amplitude and phase at eachscanning position are recorded, and can be Fourier transformed toproduce patterns of antenna radiation.

[0003] However, this known method suffers from the drawback that thecost of the network analyser is very high, and the network analysers arerestricted in frequency, as a result of which the range of applicationsof the method is fairly limited.

[0004] Preferred embodiments of the present invention seek to overcomethe above disadvantages of the prior art.

[0005] According to an aspect of the present invention, there isprovided a microwave holographic measuring method comprising:

[0006] providing a first electrical signal of at least one microwavefrequency;

[0007] directing a first part of said first signal to a first antenna;

[0008] applying predetermined changes of phase and amplitude to a secondpart of said first signal to produce a second electrical signal, whereinsaid second part is coherent with said first part;

[0009] detecting microwave radiation at a plurality of locations bymeans of a second antenna to generate a respective third electricalsignal at each said location; and

[0010] combining said second and third signals to produce a fourthelectrical signal.

[0011] By applying predetermined changes of phase and amplitude to partof the first electrical signal, this provides the advantage that thesecond electrical signal can reproduce the behaviour of referencemicrowave radiation which would interfere with the radiation emitted bythe antenna under test, i.e. by electrically imitating the behaviour ofinterfering microwave radiation. As a result, a hologram of theradiation pattern observed at the sampling antenna can be produced,which avoids the necessity for a network analyser. This reduces the costof equipment for carrying out the method, and also enables the apparatusto operate over a wider range of frequencies or even simultaneousoperation at multiple frequencies, which in turn broadens the range ofapplications of the method. The invention also has the advantage that byelectrically imitating the behaviour of interfering microwave radiation,this under some circumstances permits the electrical synthesis ofreference microwave radiation which cannot be produced in the form ofmicrowaves.

[0012] The predetermined changes of phase and amplitude may be chosen toreproduce a predetermined microwave signal at each said location.

[0013] The method may be a method of measuring radiation characteristicsof the first antenna.

[0014] The method may further comprise the step of comparing values ofsaid fourth electrical signal with corresponding values of the fourthelectrical signal for an antenna of known characteristics.

[0015] This provides the advantage of enabling the properties ofantennae to be tested by comparing unprocessed image holographic data,as opposed to processed data, thus offering savings in efficiency.

[0016] In a preferred embodiment, the method is a method of forming amicrowave image of an object, and further comprises the step ofilluminating the object with said first part.

[0017] In a preferred embodiment, the predetermined changes of phase andamplitude are chosen to reproduce a predetermined microwave signal ateach said location.

[0018] The method preferably further comprises the step of combiningsaid fourth signal with a signal representing predetermined microwaveradiation to generate holographic image data for each said location.

[0019] The method may further comprise the step of applyingpredetermined changes of phase and amplitude to said fourth electricalsignal to produce a fifth electrical signal, wherein said fifthelectrical signal represents a microwave image of the object.

[0020] The method may further comprise the step of processing saidfourth electrical signal to generate data representing an image of theobject at locations other than said plurality of locations.

[0021] The method may be a method of detecting buried objects.

[0022] According to another aspect of the present invention, there isprovided a microwave holographic measuring apparatus comprising:

[0023] signal generator means for generating a first electrical signalof at least one microwave frequency;

[0024] coupler means for directing a first part of said first signal toan antenna and providing a second part of said first signal, whereinsaid second part is coherent with said first part;

[0025] phase/amplitude adjusting means for applying predeterminedchanges of phase and amplitude to said second part to produce a secondelectrical signal;

[0026] at least one first antenna for detecting microwave radiation at aplurality of locations to generate a respective third electrical signalat each said location; and

[0027] combining means for combining said second and third signals toproduce a fourth electrical signal.

[0028] The apparatus may further comprise at least one second antennafor receiving said first part of said first signal and illuminating anobject.

[0029] The apparatus may further comprise detector means for detectingsaid fourth signals.

[0030] The apparatus preferably further comprises position adjustingmeans for adjusting the position of the or each said second antenna.

[0031] The apparatus may further comprise controller means forcontrolling said phase/amplitude adjusting means.

[0032] Preferred embodiments of the invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:

[0033]FIG. 1 is a schematic view of a prior art antenna testingapparatus;

[0034]FIG. 2 is a schematic view of an antenna testing apparatusembodying the present invention; and

[0035]FIG. 3 is a schematic view of a microwave imaging apparatusembodying the present invention.

[0036] Referring to FIG. 2, an apparatus 100 for measuring the radiationcharacteristics of an antenna 101 (or for determining any faults inantenna 101) has a microwave source 102 for producing an electricalsignal of one or more microwave frequencies. The signal S from microwavesource 102 is fed to a directional coupler 103 or other signal splittingdevice which directs signal S1, part of signal S, to the antenna 101under test, and a signal S2, also part of signal S and coherent withsignal S1, to a variable attenuator 104. The variable attenuator 104 iscontrolled by a computer 105 and alters the amplitude of signal S2 toproduce signal S3. Signal S3 is fed to a phase shifter 106, alsocontrolled by computer 105, for adjusting the phase of signal S3 toproduce signal S4.

[0037] A sampling antenna 107 is mounted to a position controller 108,which can move the antenna 107 along the axes X and Y and which is alsocontrolled by computer 105. It will be appreciated by persons skilled inthe art that the sampling antenna 107 can also be moved in theZ-direction, for example for carrying out object imaging applications.The sampling antenna 107 detects the microwave radiation from theantenna 101 under test at various positions to which it is moved by theposition controller 108, and the detected radiation signal S5 is passedto a mixing device such as a hybrid tee 109, to which the signal S4 isalso fed. The signals S5 and S4 interfere with each other in the hybridtee 109 to produce an intensity pattern which is detected by diodedetector 110 to output a sampled intensity pattern.

[0038] The operation of the apparatus shown in FIG. 2 will now bedescribed.

[0039] The signal S4 input to the hybrid tee 109 has selected amplitudevariations applied to it via variable attenuator 104 and phase changesapplied by phase shifter 106, both of which are under the control ofcomputer 105. Because the signals S4 and S5 are coherent with eachother, the signals interfere with each other in hybrid tee 109 and asampled intensity pattern is output from diode detector 110.

[0040] The amplitude and phase changes applied by variable attenuator104 and phase shifter 106 can be chosen by computer 105 to reproduce apredetermined wave at the position of sampling antenna 107, for examplea plane wave, with the result that the signal output from hybrid tee 109represents the output of antenna 101 under test interfered with acoherent reference wave. A second example of the predetermined wave thatcan be simulated is a spherical wave, for example when the focus of aspherical reference wave is located in the plane of the antenna undertest, which would otherwise prevent a spherical reference wave frombeing used because of the problem of two antennae being in the sameposition. In other words, the sampled intensity pattern output fromdiode detector 110 represents a hologram of the output of antenna 101.By application of Fourier transforms and/or inverse Fourier transformsand filtering to the sampled intensity pattern, antenna far-fieldradiation patterns for the antenna 101 can be produced.

[0041] Referring now to FIG. 3, in which parts common to the embodimentof FIG. 2 are denoted by like reference numerals but increased by 100,an apparatus 200 for forming a microwave holographic image of an object220 uses antenna 201 to illuminate object 220 with a microwave beamderived from signal S1. The beam reflected by object 220 is detected atvarious locations by sampling antenna 207 to produce signal S5 which isthen combined with signal S4 in hybrid tee 209. In a manner similar tothe embodiment of FIG. 2, a sampled intensity pattern is output by diodedetector 210.and because the phase and amplitude variations applied tosignal S4 represent a reference beam coherent with signal S1, thesampled intensity pattern forms a hologram of the object 220. Thesampled intensity pattern can be subject to Fourier transformation and,if appropriate, inverse Fourier transformation and filtering to producea plane wave spectrum of the object 220, and by means of manipulation byinverse Fourier transforms, a microwave image of the object can beobtained.

[0042] The hologram produced by the method of the present invention canbe processed by means of mathematical algorithms which will be known topersons skilled in the art to produce data representing the image of anobject at locations other than the locations at which samplingmeasurements are made. One application of this feature is the detectionof buried objects, such as landmines. For example, a microwave emitterand an array of detectors are arranged above an area of ground which isknown to be free of landmines, for example because it has been cleared.The microwaves received at the detectors consist of high intensitysignals representing microwaves from the emitter reflected from thesurface of the ground, and lower intensity signals representingmicrowaves reflected by objects buried under the ground. Phase andamplitude changes are then applied to the apparatus so that thedetectors have zero output in the case of the area of ground free ofmines, so that when the same microwave input radiation is applied to asimilar piece of ground, the reflection of microwaves by buriedlandmines will cause one of more of the detectors to emit a non-zerooutput.

[0043] It will be appreciated by persons skilled in the art that theabove embodiments have been described by way of example only and not inany limitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims.

1. A microwave holographic measuring method comprising: providing a first electrical signal of at least one microwave frequency; directing a first part of said first signal to a first antenna; applying predetermined changes of phase and amplitude to a second part of said first signal to produce a second electrical signal, wherein said second part is coherent with said first part; detecting microwave radiation at a plurality of locations by means of a second antenna to generate a respective third electrical signal at each said location; and combining said second and third signals to produce a fourth electrical signal.
 2. A method according to claim 1, wherein the predetermined changes of phase and amplitude are chosen to reproduce a predetermined microwave signal at each said location.
 3. A method according to claim 1 or 2, wherein the method is a method of measuring radiation characteristics of the first antenna.
 4. A method according to claim 3, further comprising the step of comparing values of said fourth electrical signal with corresponding values of the fourth electrical signal for an antenna of known characteristics.
 5. A method according to claim 1 or 2, wherein the method is a method of forming a microwave image of an object, and further comprises the step of illuminating the object with said first part.
 6. A method according to claim 5, further comprising the step of combining said fourth signal with a signal representing predetermined microwave radiation to generate holographic image data for each said location.
 7. A method according to claim 5 or 6, further comprising the step of applying predetermined changes of phase and amplitude to said fourth electrical signal to produce a fifth electrical signal, wherein said fifth electrical signal represents a microwave image of the object.
 8. A method according to any one of claims 5 to 7, further comprising the step of processing said fourth electrical signal to generate data representing an image of the object at locations other than said plurality of locations.
 9. A method according to claim 8,, wherein the method is a method of detecting buried objects.
 10. A microwave holographic measuring method substantially as hereinbefore described with reference to FIGS. 2 and 3 of the accompanying drawings.
 11. A microwave holographic measuring apparatus comprising: signal generator means for generating a first electrical signal of at least one microwave frequency; coupler means for directing a first part of said first signal to an antenna and providing a second part of said first signal, wherein said second part is coherent with said first part; phase/amplitude adjusting means for applying predetermined changes of phase and amplitude to said second part to produce a second electrical signal; at least one first antenna for detecting microwave radiation at a plurality of locations to generate a respective third electrical signal at each said location; and combining means for combining said second and third signals to produce a fourth electrical signal.
 12. An apparatus according to claim 11, further comprising at least one second antenna for receiving said first part of said first signal and illuminating an object.
 13. An apparatus according to claim 11 or 12, further comprising detector means for detecting said fourth signals.
 14. An apparatus according to any one of claims 11 to 13, further comprising position adjusting means for adjusting the position of the or each said second antenna.
 15. An apparatus according to any one of claims 11 to 14, further comprising controller means for controlling said phase/amplitude adjusting means.
 16. A microwave holographic measuring apparatus substantially as hereinbefore described with reference to FIGS. 2 and 3 of the accompanying drawings. 